Orphan Block

An orphan block is a valid blockchain block that ends up outside the main chain, either because its parent is unknown or because it lost a race to another block.

In a strict technical sense, an orphan block was a block whose parent block was unknown to the receiving node. These appeared in older Bitcoin Core versions that could accept blocks without full data about their ancestry. Since the Bitcoin Core v0.10 upgrade in early 2015, true orphan blocks are no longer possible on Bitcoin.

Stale blocks are generated when two different miners broadcast valid blocks at nearly the same time. The network briefly splits into two competing versions of the chain until one block is kept and the other is abandoned, following the rule that the longest valid chain prevails. Both blocks are valid, but only one becomes part of the main chain.

Because blocks travel between each node with a small delay, one group of nodes may adopt one block while another group adopts the competing one. Improvements in how blocks are relayed, such as compact block techniques that reduce bandwidth and latency, have made these temporary forks less frequent, so stale blocks are now quite rare on Bitcoin. Most of the time they occur naturally and by chance, though they can also appear when a malicious actor tries to build an alternate valid chain, as seen in a 51% attack.

Ethereum once had a similar concept called uncle blocks, which were valid blocks that were not part of the canonical chain. Uncle blocks were closer to Bitcoin's stale blocks than to true orphan blocks, and miners received a small reward for them to reduce the advantage of large mining operations. After Ethereum moved from proof-of-work to proof-of-stake in The Merge in 2022, uncle blocks became obsolete, since the network no longer produces blocks through mining. As a result, this term is now mainly of historical interest for Ethereum.

Learn more: What Is Blockchain and How Does It Work?